1. Field of the Invention
The present invention relates to a secondary guidance system for a print medium on a graphics printer in which a part of the print medium hangs loosely from the printer during printing. More specifically, the present invention relates to a vector mode graphics printer (pen plotter) in which a drawing is obtained by relative movement along two orthogonal directions between a print medium, such as a sheet of paper, and a writing head. The present invention is also applicable to graphics printers operating in a scanning mode (raster printer) in which the drawing is composed of points obtained by a line-by-line scanning while the print medium is driven past one or a plurality of writing heads.
2. Description of Related Technology
In the case of a pen plotter, for example, a drawing is generally produced by a to-and-fro movement of the print medium along a first direction (X), and a to-and-fro movement of the writing head along a second direction (Y), perpendicular to the X direction. The plotter can be adapted to use either a pre-cut sheet or a continuous roll. In the former case, the print plotter is supplied with pre-cut sheets of the desired formats. In the latter case, the print medium is drawn from a supply reel to obtain a length corresponding to the format of the drawing to be made, and the drawing is then reproduced by appropriately controlling the displacements of the print medium and the writing head.
During printing, the print medium is mainly guided by drive means acting along the X direction, e.g., a drum against which the print medium is applied by means of pressure rollers. In the vicinity of the writing zone the print medium passes over a platform that may be extended towards the front and/or the rear by a skirting. The platform may be provided with openings that communicate with a suction means in order to create a partial vacuum beneath the print medium in the zone where the printing instrument is active. These guide means ensure that the print medium is properly supported at the level of the writing zone, in order to prevent it from lifting from the platform or to start vibrating as a result of the movements imparted to it. However, once the print medium is no longer in contact with these guiding means, it hangs freely from the graphics printer.
In the case of a continuous print medium drawn from a supply reel, the remanence of the rolled state produces a tendency for the free end of the medium, hanging at the front of the graphics printer, to roll up on itself, thereby forming a loop. Accordingly, as the printed face of the print medium curls towards the rear, it can come into contact with a part of the printer or any object supported by the printer, causing the drawing to be smeared.
This risk of smearing is particularly high if the printer is equipped with an automatic storage device for the drawings. Such a device generally comprises a horizontal bar (which can be part of the printer stand) located at a lower level and slightly recessed with respect to the print medium outlet, which is at the front of the printer. The rubbing action between the loop formed at the end of the print medium on which a drawing is being made and a stored sheet on which a drawing has already been made can cause smearing of both drawings. This is all the more likely to occur if the print medium is made of a non-absorbing material, such as a sheet of polymer, on which the ink will take a long time to dry.
The contact between the loose end of the print medium and a part of the printer or an object supported by the printer can create a non-negligible sound disturbance in the immediate environment of the printer, since the medium is generally displaced by rapid to-and-fro movements with very high accelerations (up to 6 g in ultra-high-speed machines). These movements are transmitted to the hanging portion of the medium and tend to cause that portion to oscillate, thereby generating noise on contact. This is all the more of a nuisance if it is the free edge of the medium that establishes the contact.
The prior art approaches to solving this problem rely solely on a mechanical secondary guiding means to steer the hanging portion of the print medium away from the printer. These means are, e.g. bars, rollers or other guides arranged in various ways to maintain the sheet in a non-contacting position, by acting on its unprinted surface. Accordingly, all these known solutions turn out to be relatively complex from a mechanical point of view. Moreover, they add to the overall dimensions of the printer, at least in its operating configuration. Furthermore, these mechanisms prove to be noisy owing to the displacement speeds of the print medium and the snatching movements produced by the high accelerations.